Oracle 碎片 优化 一例
一个客户打过来的，说是碰到一个很奇怪的问题。在一张上千万记录的大表里，做一个SELECT * FROM lt;TAB_NAMEgt; WHERE ROWNUM
一个客户打过来的，说是碰到一个很奇怪的问题。在一张上千万记录的大表里，做一个SELECT * FROM call count cpu elapsed disk query currentrows ------- ------ -------- ---------- -------------------- ---------- ---------- Parse 1 0.14 0.17 44 198 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 8 3.71 5.86 67489 68340 0 99 ------- ------ -------- -------------------- ---------- ---------- ---------- total 10 3.85 6.03 67533 68538 0 99 从这上面看，确实产生了67533个物理读和68538个逻辑读。执行时间为6.03秒。从等待事件来看： BINDS #39: EXEC #39:c=0,e=88,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,tim=1422207486718 WAIT #39: nam='SQL*Net message to client'ela= 7 driver id=1650815232 #bytes=1 p3=0 obj#=206418 tim=1422207486810 WAIT #39: nam='SQL*Net more data to client'ela= 203 driver id=1650815232 #bytes=2002 p3=0 obj#=206418 tim=1422207487071 WAIT #39: nam='SQL*Net more data to client'ela= 66 driver id=1650815232 #bytes=2020 p3=0 obj#=206418 tim=1422207487175 WAIT #39: nam='db file scattered read' ela=515 file#=146 block#=92900 blocks=5 obj#=206418 tim=1422207488208 WAIT #39: nam='db file scattered read' ela=918 file#=146 block#=92905 blocks=8 obj#=206418 tim=1422207489579 WAIT #39: nam='db file scattered read' ela=2121 file#=146 block#=92914 blocks=7 obj#=206418 tim=1422207492091 WAIT #39: nam='db file scattered read' ela=617 file#=146 block#=92921 blocks=8 obj#=206418 tim=1422207493135 WAIT #39: nam='db file scattered read' ela=493 file#=146 block#=92930 blocks=7 obj#=206418 tim=1422207494016 WAIT #39: nam='db file scattered read' ela=1666 file#=147 block#=897417 blocks=8 obj#=206418 tim=1422207496049 WAIT #39: nam='db file scattered read' ela=1026 file#=147 block#=897426 blocks=7 obj#=206418 tim=1422207497350 WAIT #39: nam='db file scattered read' ela=378 file#=147 block#=897433 blocks=8 obj#=206418 tim=1422207498049 WAIT #39: nam='db file scattered read' ela=1075 file#=147 block#=897442 blocks=7 obj#=206418 tim=1422207499416 WAIT #39: nam='db file scattered read' ela=1649 file#=147 block#=897449 blocks=3 obj#=206418 tim=1422207501237 WAIT #39: nam='db file scattered read' ela=2768 file#=147 block#=897453 blocks=4 obj#=206418 tim=1422207504191 WAIT #39: nam='db file scattered read' ela=653 file#=147 block#=897458 blocks=7 obj#=206418 tim=1422207505141 WAIT #39: nam='db file scattered read' ela=1588 file#=147 block#=897465 blocks=8 obj#=206418 tim=1422207507029 WAIT #39: nam='db file scattered read' ela=460 file#=147 block#=897474 blocks=7 obj#=206418 tim=1422207507787 WAIT #39: nam='db file scattered read' ela=608 file#=147 block#=897481 blocks=8 obj#=206418 tim=1422207508697 WAIT #39: nam='db file scattered read' ela=564 file#=147 block#=897490 blocks=7 obj#=206418 tim=1422207509571 WAIT #39: nam='db file scattered read' ela=832 file#=147 block#=897497 blocks=8 obj#=206418 tim=1422207510668 WAIT #39: nam='db file scattered read' ela=846 file#=148 block#=102411 blocks=16 obj#=206418 tim=1422207512030 WAIT #39: nam='db file scattered read' ela=4872 file#=148 block#=102427 blocks=16 obj#=206418 tim=1422207517488 WAIT #39: nam='db file scattered read' ela=1624 file#=148 block#=102443 blocks=16 obj#=206418 tim=1422207520062 确实存在大量的DB FILE SCATTERD READ。这更加坚信了我的观点，表里存在大量的碎片。找第一个SCATTERD READ的参数 file#=146 block#=92900，让客户执行alter system dump datafile 146 block min 92900 block max 92904。
获得的结果如下：
data_block_dump,data header at0x6000000000208e64
===============
tsiz: 0x1f98
hsiz: 0x4c
pbl: 0x6000000000208e64
bdba: 0x24816ae4 76543210
flag=--------
ntab=1
nrow=29
frre=0
fsbo=0x4c
fseo=0xf7
avsp=0x1f4c
tosp=0x1f4c
0xe:pti[0] nrow=29 offs=0
0x12:pri[0] sfll=1
0x14:pri[1] sfll=2
0x16:pri[2] sfll=3
0x18:pri[3] sfll=4
0x1a:pri[4] sfll=5
0x1c:pri[5] sfll=6
0x1e:pri[6] sfll=7
0x20:pri[7] sfll=8
0x22:pri[8] sfll=9
0x24:pri[9] sfll=10
0x26:pri[10] sfll=11
0x28:pri[11] sfll=12
0x2a:pri[12] sfll=13
0x2c:pri[13] sfll=14
0x2e:pri[14] sfll=15
0x30:pri[15] sfll=16
0x32:pri[16] sfll=17
0x34:pri[17] sfll=18
0x36:pri[18] sfll=19
0x38:pri[19] sfll=20
0x3a:pri[20] sfll=21
0x3c:pri[21] sfll=22
0x3e:pri[22] sfll=23
0x40:pri[23] sfll=24
0x42:pri[24] sfll=25
0x44:pri[25] sfll=26
0x46:pri[26] sfll=27
0x48:pri[27] sfll=28
0x4a:pri[28] sfll=-1
block_row_dump:
end_of_block_dump
里面全部是空块。建议客户做一个ALTER TABLE
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